In discussing the pattern of recruitment in regards to muscle fiber type with increasing intensity, a runner could be an example of providing increasing intensities throughout the activity. As an example, as a runner begins his run, and the goal is a marathon, that runner starts at a moderate to slow pace(a jog), in which, type 1 muscles fibers provide the abundance of muscular contraction during this beginning and low intensity phase. Powers and Howley state that approximately up to 40% of VO2 output generates the activity of type 1. Type 1 fibers (slow twitch) require a large amount of oxygen for the muscle to generate Adenosine Triphosphate (ATP) from carbohydrates and fat. At this pace, the runner could potentially keep continual muscle contraction in excess of 1-4 hours.
Continual supply of glucose to aid in ATP production will be generated from the glycogen(stored form of carbohydrates) stored in the muscle and liver, as well as fatty acids will be a slower burning energy source. If this runner plans to run a shorter race, lasting approximately 20 to 60 minutes, such as a 5-10k race, the runner could possibly engage in taking longer strides that require greater force production of the hamstrings and gluteus maximus, which the runners’ heart rate will continue to rise, requiring more oxygen being shuttled to the working muscle. This increase demand could possibly increase the runners VO2, dependent on the runners bioenergetic(energy system) and/or biomechanical(muscle fibers) factors(Powers and Howley). The determining factor of the runner keeping this faster pace would be set by the accumulation of lactacte in the muscle, or the lactacte threshold.
The same runner taking part in a possibly a 1 mile run that could last from 4-6 minutes, would now increase the intensity of the run and increase their VO2 to a closer max level, approximately upwards of 90%. At this point, Type IIa muscles fibers will begin to contract to meet the increased demands of the faster and longer strides. Continuously, both type 1 and type IIa are contracting through a process called aerobic glycolysis(breaking down of glycogen through the use of oxygen). Type IIa have a higher mitochondrial density (lower than type 1 however), but a higher force output than the type 1 fiber. As a result of type 1 and type IIa fibers being recruited, lactate and hydrogen will be increased in the muscle creating a faster onset of fatigue. If the runner partakes in a sprint, possibly a 100 meter dash, which may last under 10 seconds, which require the greatest amount of force output, the runner now forces the Type IIx fast-twitch fiber to contract to meet the intensity demands. The Type IIx fiber has a low mitochondrial density in comparison to the type 1 and type IIa, so is susceptible to faster levels of fatigue, however provides the greatest force output compared to the others. This fiber type allows for ATP supply without the presence of oxygen(anaerobic), and will result in the muscle requiring ATP utilization faster than ATP production, which greatly enhances the levels of lactate which lead to a faster outcome of fatigue. ATP production during this phase is generated through the ATP-PC (-Phosphocreatine) system, which relies on the existing stores of ATP and Phosphocreatine in the working muscles. This source of ATP production can typically only last between 10-15 seconds, depending on the amount of force output.
POWERS, S. K. AND HOWLEY, E. T.
2012 – McGraw-Hill Higher Education – New York, NY
In-text: (Powers and Howley, 2012)
Your Bibliography: Powers, S. and Howley, E. (2012). Exercise physiology. New York, NY: McGraw-Hill Higher Education.
A person who consists predominantly of these muscle fibers would have a more lean or narrow muscular presence, and would find engaging in longer duration activities at a lower intensity a more suitable choice. Activities such as long distance running, ie, 10k, 5k or even marathons and half marathons.
A person consisting of Type II muscle fibers would typically carry a little larger muscular frame, and find that activities that engage in moderate to higher intensity with a shorter time frame would be most suitable. Such activities as most higher intensity sports like soccer, football, and mid-distance sprinting.
A person consisting primarily of this muscle fiber would carry a larger, more muscular frame and have a higher than normal strength output. This person would be well suited for short duration, high intensity activities such as weight training, power lifting, short distance sprinting and the like.